摘要:

Diffusion coefficients of the chloride ion in NaCl have been measured between 300° and 700°C using an isotopic exchange method. The concentrations of cation impurities were deduced from electrical conductivity measurements over the same temperature range. In the extrinsic region, the diffusion coefficients do not decrease with increasing amounts of cation impurity to the extent required for diffusion by single vacancies.The general conclusion that diffusion occurs both by single vacancies and by vacancy pairs agrees with that reached earlier by Laurance. Analyses of the experimental results in the intrinsic region yield for single vacancy anion diffusionDa=1.1×e−1.92/kTcm2sec−1and for vacancy pair diffusionDp=363×e−2.37/kTcm2sec−1, and the activation energies, which are given in electron volts, agree well with theoretical estimates.Large variations were observed in the anion diffusion coefficients for crystals doped with Ca++; the cause cannot be established definitely.

ISSN:0021-8979    

DOI:10.1063/1.1714040

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The stability of the shape of an infinite cylinder undergoing radial growth controlled by diffusion is studied by a method originated by Mullins and Sekerka (MS). It is found that the circular cross section of a cylinder is stable when its radius is less than and unstable when its radius is greater than a certain radiusRc. This result is analogous to the MS result that a sphere is stable below and unstable above a certain radiusRc, which is seven times the critical radiusR* of nucleation theory. However, in the present case, the ratioRc/R* is not equal to seven, but is a function ofS=(c∞‐cs)/(C‐cs), wherec∞, cs, andCare the concentrations of the solute at infinity, at the surface of the cylinder, and in the precipitate, respectively.The case of perturbations in the radius along the length of the cylinder is also treated. Potential application of the result to such problems as the growth of branches on dendrites is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1714041

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The theory developed in Part I is applied to the interaction energy between vacancies and dislocations for the case of elastic isotropy. In the elastic stress fields of dislocations, outside the dislocation cores, the vacancies are attracted into regions of compressional strain, as expected from the analogy with the Cottrell interaction between dislocations and substitutional atoms of a size smaller than the matrix atoms. Within dislocation cores, strains are much larger than in the elastic regions, but stresses are only moderately higher. Consequently, the two contributions to the binding energy which depend on normal strains rather than on normal stresses, namely, the electronic interaction energy and the change in the vibrational entropy, are much more important within cores than outside them and repel vacancies from regions of compressional strain. With the best possible estimates of core stresses and strains it is found that the electronic term is dominant in the cores at low temperatures, and the entropy term at high temperatures; therefore, vacancies are attracted into the dilated regions of dislocation cores with an interaction energy which rises linearly with the temperature.In near‐screw dislocations and in a range of mixed dislocations, the nominally ``compressed'' regions of the core are in fact dilated, because non‐Hookean behavior, superimposed on the symmetric strains which may be extrapolated from conditions existing outside the core, results in an over‐all dilatation. The interaction energy between vacancies and the cores of screw dislocations can therefore be significant, even though the contribution to it arising from the reduction of stored strain energy is small, and no Cottrell‐type interaction energy exists. According to the above results, climb of edge dislocations and of a range of mixed dislocations is more difficult than expected on the basis of previous theories, inasmuch as the vacancies are repelled from the most compressed region of the dislocation cores, which they must traverse if a dislocation is to climb by the absorption or emission of vacancies. This repulsion is smaller and may even be replaced by an attraction in mixed dislocations with predominant screw character, depending on the angle between the Burgers vector and the dislocation axis, and would explain why climb of edge dislocations sometimes seems to be inhibited whereas mixed and near‐screw dislocations climb. The interaction energies in the elastic stress field of dislocations at all temperatures, and in the cores at very low temperatures, is quite small, below 0.1 eV; at high temperatures in the cores, it may exceed 0.5 eV.

ISSN:0021-8979    

DOI:10.1063/1.1714042

出版商:AIP    

年代:1965

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1714043

出版商:AIP    

年代:1965

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1714044

出版商:AIP    

年代:1965

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1714045

出版商:AIP    

年代:1965

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1714046

出版商:AIP    

年代:1965

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1714047

出版商:AIP    

年代:1965

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1714048

出版商:AIP    

年代:1965

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1714049

出版商:AIP    

年代:1965

数据来源: AIP